Offering many advantages such as high productivity, high design flexibility, light weight, rustproofing, high impact resistance, etc., plastics are used these days in a wide variety of applications for automotive parts, electronic parts, building materials and the like. In particular, since polyolefin resins are inexpensive and have excellent moldability, chemical resistance, water resistance, impact resistance, and electrical properties, they have been widely used as industrial materials and are one of the types of material for which demand is expected to further increase in the future.
However, unlike acrylic resins and similar synthetic polar resins, polyolefin resins have drawbacks due to their crystallinity and nonpolarity, i.e., it is difficult to coat or bond them. Primers containing weakly chlorinated polyolefins that are chlorinated to 10–40 wt. % as a binder have heretofore been used to coat or bond such low-adhesion polyolefin resins.
When the cost of transportation or the amount of solvent used is considered, primers should preferably have a large solids content. However, an excessive solids content results in decreased flowability or increased viscosity at low temperatures, adversely affecting primers in practical applications, for example, impaired solubility in coatings and workability during spray coating. Moreover, the storage stability at low temperatures is also adversely affected. To control viscosity, if the molecular weight of the resins is lowered by degradation or like methods, or low-molecular-weight rosin ester resins are added, the initial adhesion and gasohol resistance are significantly impaired.
Recently, the coating technique typically employed to reduce coating lines or costs is wet-on-wet coating in which primers and coatings are applied sequentially and baked only once. However, with such coating methods, it has been impossible to obtain sufficient adhesion particularly when melamine coatings are used. This is presumably because intermediate layers are created between primers and melamine coatings due to the high polarity of melamine resins and the low polarity of conventional primers. Therefore.,resin compositions for high polarity primers are required.